Closed-loop mimo systems and methods

ABSTRACT

Methods, devices and systems are provided for transmitting and receiving MIMO signals. Transmitting of the MIMO signals involves pre-coding each of at least two data symbols using a respective pre-coding codeword to preclude a corresponding plurality of pre-coded data symbols. A respective signal is transmitted from each of a plurality of antennas, the respective signal including one of the pre-coded signals and at least one pilot for use in channel estimation. The signals collectively further include at least one beacon pilot vector consisting of a respective beacon pilot per antenna, the beacon pilot vector containing contents known to a receiver for use by the receiver in determining the codeword used to pre-code the at least one data signal. Receiving of the MIMO signals involves receiving a MIMO signal containing data symbols pre-coded with a codeword. The MIMO signal includes pilots, and including at least one beacon pilot vector containing contents known to a receiver/each beacon pilot vector containing one symbol from each transmit antenna. Processing is performed on the at least one beacon pilot vector to determine which codeword was used to pre-code the data symbols.

RELATED APPLICATION

This application claims the benefit of prior U.S. ProvisionalApplication No. 60/783,711 filed Mar. 17, 2006, hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to closed loop MIMO (Multiple InputMultiple Output) techniques, more specifically closed loop MIMOtechniques involving pre-coding.

BACKGROUND OF THE INVENTION

Closed-loop MIMO aims to significantly improve throughput andreliability of nomadic user equipment (UEs).

Pre-coding is proposed in UMTS (Universal Mobile TelecommunicationsSystem) LTE (Long Term Evolution) as one of the main approaches forclosed-loop MIMO. Systems that employ pre-coding multiply a data symbolvector {right arrow over (s)} containing one element per transmitantenna by a pre-coding matrix F prior to transmission. The receivedsignal is defined by {right arrow over (r)}=HF{right arrow over(s)}+{right arrow over (n)}, where H is the channel matrix, and {rightarrow over (n)} is noise. For a system with two transmit antennas andtwo receive antennas, {right arrow over (r)}, {right arrow over (s)},{right arrow over (n)} are vectors with two elements each, B is a 2×2matrix, and F is a 2×2 matrix. The pre-coding matrix F is chosen from agroup of predefined matrixes that is called a codebook {F}. In somecases the receiver tells the transmitter which pre-coding matrix to use.For FDD (frequency division duplex) air interfaces the informationidentifying a pre-coding matrix may be fed back through either channelsounding approaches or codebook index approaches. TDD (time divisionsystems) may also use the codebook based approach. A detailed example ofin approach to pre-coding for MIMO transmission is described in it. J.Love, et al, “Limited Feedback Unitary Pre-coding for SpatialMultiplexing Systems”, IEEE Trans. Inf. Theory, vol. 51, no. 8, pp.2967-2976, August 2005.

Codebook index feedback involves the receiver signalling to thetransmitter an index of which pre-coding matrix to use (so-calledcodeword index). There are a plurality of indexes each corresponding toa respective pre-coding matrix. One problem, however, is that codebookindex feedback approaches use a large amount of uplink radio resources.

Another problem occurs for common pilot based pre-coding schemes. In acommon pilot based scheme, the pilots are not pre-coded, and hence havedifferent channel matrices from the data. If the receiver knows thepre-coding matrices used by the transmitter in such scenarios, it canreconstruct the channel matrix with pre-coding effects. When the datachannel matrix can be correctly reconstructed, data can be correctlydecoded. If, however, the receiver uses a different pre-coding matrixfrom the transmitter, the constructed effective data channel matrix willbe wrong. As a result detected data will be useless due to incorrectchannel references. In this case the incorrectly detected data cannot beused for H-ARQ purposes either.

An example of the common pilot approach is shown in FIG. 1 for a twotransmit antenna case. In FIG. 1 (and other Figures described below),the horizontal axis 210 is frequency (OFDM (Orthogonal FrequencyDivision Multiplexing) sub-carriers) and the vertical axis 212 is time(OFDM symbols). Each small circle represents a transmission on aparticular sub-carrier over a particular OFDM symbol duration. Inlocations 214, pilots are transmitted by a first transmit antenna Tx-1,and in locations 216, pilots are transmitted by a second transmitantenna Tx-2. Remaining locations are available for data transmission byboth antennas. In the illustrated example, data includes pre-coded data218 for a first UE (UE-1), and pre-coded data 220 for a second UE(UE-2). Typically, the pre-coding applied for pre-coded data 218 will bedifferent from that applied for pre-coded data 220. With the commonpilot approach, the same pilots are used for both UEs and hence theycannot be pre-coded.

In some existing approaches, the codeword index is fed back in adifferential manner. In other words the difference between a currentindex and a previous index is fed back rather than the codeword indexper se. Another problem with common pilot based closed-loop MIMO schemesrelates to errors that occur in the transmission of the differentialcodeword index. Closed-loop MIMO is typically intended for nomadic UEsfor whom channel conditions will not change very quickly. For each givencurrently used pre-coding matrix, a small subset of possible nextcodewords is defined, for example those that would most likely beselected from the full set due to slow channel variation. This subset ofcodewords can be determined in several ways, such as spatial correlationand matrix correlation. If an index associated with this subset is fedback rather than an index from the entire set of codewords, fewer bitsare needed. However, if a feedback error occurs the transmitter willkeep using the wrong subsets for subsequent pre-coding updates. That isto say feedback error propagates in differential codeword indexfeedback. Consider the following example. Suppose that the transmitterand receiver are synchronized at the beginning, and both use codewordV₀. Now with the differential codebook index feedback, the new codewordwould be V₁, which is in the small differential codebook associated withV₀. Since an error occurred, the transmitter thinks that it should useV₂, which is also from the small codebook associated with V₀. For thenext feedback, the receiver will feed back a differential indexassociated with V₁, but the transmitter will get the new codeword fromthe differential codebook associated with V₂, and this goes on and on.This is how error gets propagated.

To address this error propagation problem, it has been proposed toperiodically reset by transmitting a whole codeword index to correctpossible feedback errors. However, this raises more problems than itsolves. To begin, the whole codeword can be wrong, and hence errorpropagates. The resetting is a random process: the reset can begindespite no errors occurring, and may not be done when an error doesoccur. To make the probability of error propagation low, a large numberof resets is required and this quickly diminishes the benefit ofdifferential codeword index feedback. To completely eliminate errorpropagation, a reset is performed each time, in which case it will ceaseto be a differential index feedback approach.

Dedicated pilot based schemes suffer shortcomings as well. In adedicated pilot based scheme, pilots can be pre-coded, and hence havethe same channel matrices as data. One problem, however, is that sinceeach UE trying to communicate with a base transceiver station (BTS) doesnot know what pre-coding matrix is being used by other UEs, the UE isunable to monitor the channel. More specifically, they do not know whichpre-coding matrix is being used, do not know the rank of the currentchannel, cannot estimate per-layer based signal to interference noiseratio (SINR), and are unable to do channel dependent scheduling, to namea few examples.

An example of the dedicated pilot approach is shown in FIG. 2 for a twotransmit antenna case. In locations 222,224, dedicated pilots specificto the first UE are transmitted by a first transmit antenna Tx-1 andsecond antenna Tx-2 respectively. In locations 226,228, dedicated pilotsspecific to the second UE are transmitted by a first transmit antennaTx-1 and second antenna Tx-2 respectively. Remaining locations areavailable for data transmission by both antennas. In the illustratedexample, data includes pre-coded data 230 for a first UE (UE-1), andpre-coded data 232 for a second UE (UE-2). Typically, the pre-codingapplied for pre-ceded data 230 will be different from that applied forpre-coded data 230. With the dedicated pilot approach, different pilotsare used for each UE in the sense that they are pre-coded using the sanepre-coding matrix as used for the data for each user.

Since both pilot and data go through the same channel, a dedicated pilotscheme is more resilient to codeword index feedback error than commonpilot based schemes. Feedback causes performance degradation, but thedecoded data can still be used for H-ARQ.

As indicated above, for dedicated pilot based approaches differentialfeedback error propagates, but the consequences are less severe than inthe common pilot case for the UE of interest. However, some of thebenefit of pre-coding with all the uplink feedback is lost. Since nowthe pre-coding matrix is wrong, which is equivalent to a random matrix,HF will have the same properties as H. This means pre-coding isequivalent to no pre-coding, and hence the benefit is lost. When thepre-coding matrix becomes random, the system behaves like an open loopsystem. Finally, a wrong per-layer based power allocation can furtherhamper system performance.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a method oftransmitting comprising: pre-coding each of at least two data symbolsusing a respective pre-coding codeword to produce a correspondingplurality of pre-coded data symbols; transmitting a respective signalfrom each of a plurality of antennas, the respective signal comprisingone of the pre-coded signals and at least one pilot for use in channelestimation; the signals collectively further comprising at least onebeacon pilot vector consisting of a respective beacon pilot per antenna,the beacon pilot vector containing contents known to a receiver for useby the receiver in determining the codeword used to pre-code the atleast one data signal.

In some embodiments, each transmitted signal is an OFDM signal, andwherein for each antenna the respective signal comprises a null in eachsub-carrier and time location used to transmit the at least one pilot inthe respective signal of each other antenna.

In some embodiments, the method further comprises pre-coding the pilotsfor use in channel estimation for transmission; wherein the at least onebeacon pilot vector is transmitted without pre-coding.

In some embodiments, the pilots for use in channel estimation aretransmitted without pre-coding; the at least one beacon pilot vector istransmitted with pre-coding.

In some embodiments, the method further comprises receiving feedbackindicating which pre-coding codeword to use.

In some embodiments, the feedback comprises a differential codewordindex.

In some embodiments, the method comprises transmitting to a plurality ofreceivers with frequency division duplex (FDD) or time division duplex(TDD) separation between content of different receivers; whereinpre-coding comprises using a respective pre-coding codeword for eachreceiver; the at least one pilot comprises a respective at least onepilot dedicated to each receiver that is pre-coded using the samecodeword as the data for that receiver.

In some embodiments, the method comprises transmitting to a plurality ofreceivers with FDD or TDD separation between content of differentreceivers; wherein pre-coding comprises using a respective pre-codingcodeword for each receiver; the at least one pilot comprise pilots thatare for use by all receivers.

In some embodiments, the method is performed for each of a plurality ofFDD or TDD MIMO radio resources.

In some embodiments, the FDD or TDD MIMO radio resources are OFDMresources.

In some embodiments, said FDD or TDD MIMO radio resource is singlecarrier based.

According to another aspect of the invention, there is provided a methodof receiving comprising: receiving a MIMO signal containing data symbolspre-coded with a codeword, the MIMO signal including pilots, andincluding at least one beacon pilot vector containing contents known toa receiver, each beacon pilot vector containing one symbol from eachtransmit antenna; processing the at least one beacon pilot vector todetermine which codeword was used to pre-code the data symbols.

In some embodiments, the method further comprises determining if thedetermined codeword was a codeword expected to be used.

In some embodiments, the method further comprises comparing thedetermined codeword with an expected codeword; if there is a matchbetween the determined codeword and the expected codeword, determiningthere is no error in codeword feedback, and performing decoding.

In some embodiments, the pilots are not pre-coded and the a least onebeacon pilot vector is pre-coded, and wherein processing the at leastone beacon pilot vector to determine which codeword was used to pre-codethe data comprises: performing channel estimation using the pilots toproduce channel estimates; using the known contents of the at least onebeacon pilot vector, and the channel estimates to determine whichcodeword was used.

In some embodiments, the pilots are also pre-coded and the at least onebeacon pilot vector is not pre-coded, and wherein processing at leastone beacon pilot vector to determine which codeword was used to pre-codethe data comprises: performing channel estimation using the pre-codedpilots to produce channel estimates; using the known contents of the atleast one beacon pilot vector, and the channel estimates to determinewhich codeword was used.

In some embodiments, the method further comprises transmitting feedbackindicating which codeword to use.

In some embodiments, the method further comprises comparing thedetermined codeword with the codeword indicated by the feedback todetermine if there has been a pre-coding codeword feedback error.

In some embodiments, the feedback comprises a differential codewordindex.

In some embodiments, the method further comprises tracking a channel ofother receivers by processing the at, least one un-preceded beacon pilotvector and pre-coded pilots of other receivers.

In some embodiments, the method further comprises upon detecting thatthere is no pre-coding codeword feedback error, using received data forH-ARQ purposes; upon detecting that there is a pre-coding codewordfeedback error, using the determined codeword in place of the codewordindicated by the feedback.

In some embodiments, transmitting the differential codeword feedbackbased on the codeword determined using the at least one beacon pilotvector.

In some embodiments, the method comprises in a transmitter, executingthe method of transmitting as described in embodiments above; in areceiver: receiving a MIMO signal containing data symbols pre-coded witha codeword, the MIMO signal including pilots, and including at least onebeacon pilot vector containing contents known to a receiver, each beaconpilot vector containing one symbol from each transmit antenna;processing the at least one beacon pilot vector to determine whichcodeword was used to pre-code the data symbols.

According to yet another aspect of the invention, there is provided atransmitter that executes methods of transmitting a described above.

According to yet a further aspect of the invention, there is provided areceiver that executes methods of receiving as described above.

According to yet another aspect of the invention, there is provided amethod comprising: provisioning a frequency division duplex MIMO radioresource for facilitating detection of feedback errors; where saidresource includes at least one pilot for each transmit antenna; andwhere said provisioning includes pre-coding a known signal vector whensaid pilots are not pre-coded.

According to another aspect of the invention, there is provided a systemcomprising: a controller operable to: provision a frequency divisionduplex MIMO radio resource for facilitating detection of feedbackerrors; where said resource includes at least one pilot for eachtransmit antenna; and where said controller is further operable topre-code a known signal vector when said pilots are not pre-coded.

According to another aspect of the invention, there is provided a systemcomprising: a controller operable to: provision a frequency divisionduplex MIMO radio resource for facilitating detection of feedbackerrors; where said resource includes at least one pilot for eachtransmit antenna; and where said controller is further operable toprovision a non-pre-coded known signal vectors when said pilots arepre-coded.

In some embodiments, said FDD MIMO radio resource is OFDM based.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theattached drawings in which:

FIG. 1 is an OFDM signal layout diagram for transmit signals thatinclude common pilots;

FIG. 2 is an OFDM signal layout diagram for OFDM signals containingdedicated pilots;

FIG. 3 is an OFDM signal layout diagram for an OFDM signal containingcommon pilots and pre-coded beacon pilots;

FIG. 4 is an example of equations for performing a pre-coding check;

FIG. 5 is an OFDM signal layout diagram showing an OFDM signal withdedicated pilots and non-pre-coded beacon pilots;

FIG. 6 is an example of equations for performing a pre-coding codewordcheck;

FIG. 7 is a block diagram of a cellular communication system;

FIG. 8 is a block diagram of an example base station that might be usedto implement some embodiments of the present invention;

FIG. 9 is a block diagram of an example wireless terminal that might beused to implement some embodiments of the present invention;

FIGS. 10A and 10B are block diagrams of a logical breakdown of exampleOFDM transmitter architectures that might be used to implement someembodiments of the present invention; and

FIG. 11 is a block diagram of a logical breakdown of an example OFDMreceiver architecture that might be used to implement some embodimentsof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with embodiments of the invention various closed-loop MIMOsystems and methods that may involve pre-coding index feedback aredescribed. Specifically the embodiments presented below are intended foruse in future 3GPP, 3GPP2 and IEEE 802.16 based wireless standards. Thebroader inventions set out in the summary, however, are not limited inthis regard. Furthermore, while embodiments of the invention aredescribed in the context of an OFDM air interface, the broader conceptsare not limited in this regard and are equally applicable to other airinterfaces such as the single carrier uplink air interface used for UMTSLTE, or any other FDD air interface adopting a frequency domain MIMOdetection approach.

Common Pilot Embodiment

In accordance with an embodiment of the invention, a closed-loop MIMOscheme is provided that uses common pilots in which “beacon pilotvectors” are introduced to enable a receiver to determine a pre-codingcodeword used at the transmitter, for example to enable a pre-codingcodeword check. Details of such a pre-coding check are provided below.Practically speaking this check can occur nearly instantaneously. Forpurposes of the common pilot scheme, the beacon pilot vectors arepre-coded whereas the common pilots are not.

Referring now to the detailed example of FIG. 3, the frequency axis isindicated at 210 and the time axis is indicated at 212. The figure usesa shorthand notation to show what is transmitted on two differentantennas. In locations 240, common pilots are transmitted by a firsttransmit antenna Tx-1, and in locations 241, common pilots aretransmitted by a second transmit antenna Tx-2. In the locations used forpilots by one antenna, nulls are transmitted on the other antenna. Ineach location 246, a beacon pilot vector for UE-1 is transmitted, and ineach location 248, a beacon pilot vector for UE-2 is transmitted. Eachbeacon pilot vector location includes a vector with one element perantenna transmitted simultaneously on the same sub-carrier frequency.This is contrast to the other pilot locations, in which one pilot signalis transmitted at a given time on a given sub-carrier frequency on oneantenna with other antennas transmitting nulls. Remaining locations areavailable for data to be transmitted by both antennas. In theillustrated example, data includes pre-coded data 242 for a first UE(UE-1), and pre-coded data 244 for a second UE (UE-2). Typically, thepre-coding applied for pre-coded data 242 will be different from thatapplied for pre-coded data 244. With the common pilot approach, the samepilots are used for both UEs and hence they cannot be pre-coded.

In the illustrated example, the beacon pilot vectors are embedded withinthe area used to transmit data for each of two users. The beacon pilotvectors within area 243 ace pre-coded with the same pre-coding used forthe pre-coded data 242 for UE-1, and the beacon pilot vectors withinarea 245 are pre-coded with the same pre-coding used for the pre-codeddata 244 for UE-2.

FIG. 3 shows a very specific example in which OFDM resources are usedfor two different UEs. Of course the number of UEs that can be handledin this manner is implementation specific. The number and location ofthe pilots for each of the UEs are implementation specific. While theexample has focused on a two transmit antenna case, more generally, thisis extendable to an N transmit antenna case. In some such embodiments,the pilots include respective pilots for each of the antennas. Whilegroups of two pilots (one for each transmit chain) are shown in FIG. 3,those of ordinary skill in the art will recognize that more than twopilots could be used without departing from the scope of the broaderconcepts. Similarly, the number and location of beacon pilot vectorsshown in FIG. 3 are not limiting. If more beacon pilot vectors are used,additional resources are required through better results may beachieved.

FIG. 4 shows an example of equations that clan be used to determine whatcodeword was used in the transmitter, for example so as to perform apre-coding codeword check. The equations pertain to each beacon pilotvector location, where it is assumed that the receiver knows whichpre-coding codebook it is expecting to be used for its data. In FIG. 4,H_(c) represents the channel matrix corresponding to the location of aparticular beacon pilot vector. An estimate of this can be derived fromcommon pilots. s in the transmitted beacon pilot vector that includesone component per antenna. V_(p) is the pre-coding matrix, and h isadditive noise. V_(k)=, k=1, . . . L is the pre-coding codebook. Anexpression for the received beacon pilots {right arrow over(r)}_(beacon) _(—) _(pilot) is given by the first equation 250. Thesecond equation 252 is used to determine which pre-coding matrix wasused in the transmitter from a set of L possible pre-coding matrices.Specifically, the expression inside the “argmin” operator in the righthand side of Equation 252 is evaluated for each of the L pre-codingmatrices and the pre-coding matrix V_(p) that results in the minimum ofthis value is identified. Assuming this matches what the receiver isexpecting, then the pre-coding codeword check succeeds. If this does normatch what the receiver is expecting, then there is an error.

The approach described above is applied for each of the UE, toindependently verify the respective pre-coding matrix used. Each UEneeds to verify all the pre-coding matrices intended for it. Note that aUE can have several pre-coding matrices, due to channel differences inits occupied bandwidth.

Multiple beacon pilot vectors within a feedback period can be usedjointly for the pre-coding codeword check purposes. However, a minimumof one beacon pilot in a given feedback period is needed.

Where content for multiple users is contained in received signals, thepre-coding codeword check descried above is performed at each receiver.

Dedicated Pilot Embodiment

In accordance with another embodiment of the invention, a closed-loopMIMO scheme using dedicated pilots is provided in which beacon pilotvectors are introduced to enable a pre-coding codeword check. Forpurposes of the dedicated pilot scenario, the beacon pilot vectors arenot pre-coded and the pilots are pre-coded. This enables discernment ofthe pre-coding being used by the transmitter.

Referring now to the detailed example of FIG. 5, in locations 260,262,dedicated pilots are transmitted by a first transmit antenna Tx-1 and asecond antenna Tx-2 respectively that are specific to a first UE. Inlocations 264,266, dedicated pilots are transmitted by the firsttransmit antenna Tx-1 and the second antenna Tx-2 respectively that arespecified to a second UE. In locations 272, beacon pilot vectors aretransmitted. As in the previous example, the beacon pilot vectors in agiven location consist of a vector with one element per antenna.Remaining locations are available for data to be transmitted by bothantennas. In the illustrated example, data includes pre-coded data 268for the first UE, and pre-coded data 270 for the second UE. Typically,the pre-coding applied for pre-coded data 268 will be different fromthat applied for pre-coded data 270. With the dedicated pilot approach,different pilots are used for each UE in the sense that they arepre-coded using the same pre-coding matrix as used for the data for eachuser. More specifically, the dedicated pilots 260,262 that are embeddedwithin the area 261 used to transmit pre-coded data 268 for UE 1 arepre-coded with the same pre-coding as was used for the pre-coded data68. Similarly, the dedicated pilots 264,266 for the second UE arelocated within the area 265 used to transmit pre-coded data 270 to thesecond UE, and the same pre-coding is applied to both the dedicatedpilots and the pre-coded data. No pre-coding is applied to the beaconpilot vectors 272.

FIG. 5 shows a very specific example in which OFDM resources are usedfor two different UEs. Of course the number of UEs that can be handledin this manner is implementation specific. The number and location ofthe pilots for each of the UEs are implementation specific. While theexample has focused on a two transmit antenna case, more generally, thisis extendable to an N transmit antenna case. In some such embodiments,the pilots include respective pilots for each of the antennas. Whilegroups of two pilots (one for each transmit chain) are shown in FIG. 5,those of ordinary skill in the art will recognize that more than twopilots could be used without departing from the scope of the broaderconcepts. Similarly, the number and location of beacon pilot vectorsshown in FIG. 5 are not limiting. If more beacon pilot vectors are used,additional resources are required though better results may be achieved.

FIG. 6 shows an example of a set of equations that can be used toperform a pre-coding check for the example of FIG. 5, all of whichpertain to a particular beacon pilot location. In FIG. 6, H_(c)represents the channel matrix without pre-coding. “G” is an effectivechannel matrix including both the effects of the channel (estimated fromdedicated pilots) and the preceding matrix, and {right arrow over(s)}_(beacon) _(—) _(pilot) is the transmitted beacon pilot vector. Theeffective channel matrix will thus be different for each user given thatdifferent pre-coding has been applied. V_(p) is the pre-coding matrixand {right arrow over (n)} is additive noise. An expression for thereceived beacon pilot vectors {right arrow over (r)}_(beacon) _(—)_(pilot) is given by the first equation 280. The second equation 282gives G as a function of H_(c) and V_(p) and the fourth equation 286gives H_(c) as a function of V_(p)′ and G. The third equation 284 isused to determine which pre-coding matrix was used in the transmitterfrom a set of L possible pre-coding matrixes. More specifically, theexpression inside the “argmin” operator in Equation 284 is evaluated foreach of the possible pre-coding matrices V_(k), for k=1 to L and theexpression that results in the minimum value is selected as

. Assuming this matches what the receiver is expecting, then thepre-coding codeword check succeeds. If this does not match what thereceiver is expecting, then there is an error.

The approach described above is applied for each of the UE, toindependently verity the respective pre-coding matrix used. Each UEneeds to verify all the pre-coding matrices intended for it. Note that aUE can have several pre-coding matrices, due to channel differences inits occupied bandwidth.

Once again multiple beacon pilot vectors within a feedback period can beused jointly for the pre-coding codeword check purposes. However, aminimum of one beacon pilot vector in a given feedback period is needed.

When estimation noise power is larger than the codeword quantizationdistances, an estimation error can occur. The larger the distancebetween codebook entries the smaller the probability of error. Severalmethods can be employed to make detection more reliable:

-   -   use more than one beacon pilot vector for each user (for each        sub-band for the example of FIG. 5);

track the codeword error—if the same wrong codeword is detected twice,then it is safe to assume that this specific codeword was used as anearlier pre-coding codeword; or

-   -   when in doubt, use the whole index feedback approach.

With the dedicated pilot approach, other UEs in the system can track thechannel by looking at the dedicated pilots and the unpre-coded beaconsso that they can use proper closed-loop schemes and be scheduledproperly. Since other UEs do not know V_(p), they cannot track thechannel used by pre-coding. This makes scheduling difficult, because ascheduler has no way to know in advance which UE this resource should beallocated to. To solve this problem, the other UEs can examine thenon-pre-coded beacon pilot vectors. Since the number of codewords islimited, this provides an efficient way for channel tracking.

Denoting {right arrow over (s)}_(pilot) as a known pilot vector, thenthe received signal on that particular pilot tone is given by

{right arrow over (r)} _(pilot) =H _(c) s _(pilot) +{right arrow over(n)}.

From dedicated pilots, we have G=H_(c)V_(p). Let {V_(k), k=1, . . . , L}be the pre-coding codebook; then the pre-coding codeword V_(p) used bythe BTS can be estimated as

${{\hat{V}}_{p} = {\underset{{k = 1},\ldots \mspace{14mu},L}{argmin}{{{\overset{\_}{r}}_{pilot} - {{GV}_{k}^{\prime}{\overset{\_}{s}}_{pilot}}}}^{2}}},$

where we assume that a channel matrix is U_(c)D_(c)V_(c)′ in its SVDform. After knowing V_(p), a UE will be able to estimate the currentchannel easily by computing

Ĥ_(c)=Ĝ{circumflex over (V)}_(p).

With both the dedicated pilot and common pilot embodiments describedabove feedback errors can be detected very quickly. If a received packetis still in error but the packet data error was not caused by pre-codingcodeword feedback error, the received data can be used for H-ARQpurposes. In the dedicated pilot case, even when pre-coding feedback iswrong, the received data can still be used for H-ARQ purpose. The reasonis that with dedicated pilots, the pilot channel is the same as the datachannel, and hence the reference is still correct for coherentdetection. In the common pilot case, when pre-coding feedback is wrong,and if the receiver does not know what pre-coding matrix is being usedby the transmitter, the data channel cannot be correctly reconstructed.In this case the received data will need to be discarded. However, whena feedback error occurs, if, the pre-coding matrix used by thetransmitter can be detected successfully notwithstanding the error, thenthe data channel can still be correctly reconstructed. In other words,if the receiver knows what pre-coding matrix is being used by thetransmitter, regardless of whether the feedback is correct or wrong, thereceived data can still be used. Of course, in this case, as explainedabove, the benefit of pre-coding is reduced.

In addition, in some embodiments, differential feedback is employed, andthe subsequent differential codeword index feedback is based on thecodeword currently used by the transmitter (as verified by the check),and this eliminates any error propagation instantly. That is to say,when a codeword feedback error has been determined, the index that wasused by the transmitter is known from the check, and the nextdifferential codeword feedback will be based on this index.

With reference to FIG. 7, a base station controller (BSC) 10 controlswireless communications within multiple cells 12, which are served bycorresponding base stations (BS) 14. In general, each base station 14facilitates communications using OFDM with mobile terminals 16, whichare within the cell, 12 associated with the corresponding base station14. The movement of the mobile terminals 16 in relation to the basestations 14 results in significant fluctuation in channel conditions. Asillustrated, the base stations 14 and mobile terminals 16 may includemultiple antennas to provide spatial diversity for communications.

A high level overview of the mobile terminal 16 and base stations 14 ofthe present invention is provided prior to delving into the structuraland functional details. With reference to FIG. 8, a base station 14configured according to one embodiment of the present invention isillustrated. The base station 14 generally includes a control system 20,a baseband processor 22, transmit circuitry 24, receive circuitry 26,multiple antennas 28, and a network interface 30. The receive circuitry26 receives radio frequency signals bearing information from one or moreremote transmitters provided by mobile terminals 16 (illustrated in FIG.9). A low noise amplifier and a filter (not shown) may be provided thatcooperate to amplify and remove out-of-band interference from the signalfor processing. Down conversion and digitization circuitry (not shown)will then down convert the filtered, received signal to an intermediateor baseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs) orapplication-specific integrated circuits (ASICs). The receivedinformation is then sent across a wireless network via the networkinterface 30 or transmitted to another mobile terminal 16 serviced bythe base station 14.

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by a carrier signal having a desiredtransmit frequency or frequencies. A power amplifier (not shown) willamplify the modulated carrier signal to a level appropriate fortransmission, and deliver the modulated carrier signal to the antennas28 through a matching network (not shown). Modulation and processingdetails are described in greater detail below.

With reference to FIG. 9, a mobile terminal 16 configured according toone embodiment of the present invention is illustrated. Similarly to thebase station 14, the mobile terminal 16 will include a control system32, a baseband processor 34, transmit circuitry 36, receive circuitry38, multiple antennas 40, and user interface circuitry 42. The receivecircuitry 38 receives radio frequency signals bearing information fromone or more base stations 14. Preferably, a low noise amplifier and afilter (not shown) cooperate to amplify and remove out-of-bandinterference from the signal for processing. Down conversion anddigitization circuitry (not shown) will then down convert the filtered,received signal to an intermediate or baseband frequency signal, whichis then digitized into one or more digital streams.

The baseband processor 34 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations, as will be discussed on greater detail below. Thebaseband processor 34 is generally implemented in one or more digitalsignal processors (DSPs) and application specific integrated circuits(ASICs).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, data, or control information, from thecontrol system 32, which it encodes for transmission. The encoded datais output to the transmit circuitry 36, where it is used by a modulatorto modulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). Various modulation and processing techniques available tothose skilled in the art are applicable to the present invention.

In OFDM modulation, the transmission band is divided into multiple,orthogonal carrier waves. Each carrier wave is modulated according tothe digital data to be transmitted. Because OFDM divides thetransmission band into multiple carriers, the bandwidth per carrierdecreases and the modulation time per carrier increases. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

OFDM modulation requires the performance of an Inverse Fast FourierTransform (IFFT) on the information to be transmitted. For demodulation,the performance of an Fourier Transform (FFT) on the received signal isrequired to recover the transmitted information. In practice, the IFFTand FFT are provided by digital signal processing carrying out anInverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform(DFT), respectively. Accordingly, the characterizing feature of OFDMmodulation is that orthogonal carrier waves are generated for multiplebands within a transmission channel. The modulated signals are digitalsignals having a relatively low transmission rate and capable of stayingwithin their respective bands. The individual carrier waves are notmodulated directly by the digital signals. Instead, all carrier wavesare modulated at once by IFFT processing.

In some embodiments, OFDM is used for at least the downlink transmissionfrom the base stations 14 to the mobile terminals 16. Each base station14 is equipped with n transmit antennas 28, and each mobile terminal 16is equipped with m receive antennas 40. Notably, the respective antennascan be used for reception and transmission using appropriate duplexersor switches and are so labeled only for clarity.

With reference to FIGS. 10A and 10B, a logical OFDM transmissionarchitecture is provided according to one embodiment. FIG. 10A is anexample of a dedicated pilot embodiment. FIG. 10B is an example of acommon pilot embodiment. In both cases, initially, the base stationcontroller 10 (FIG. 7) will send data to be transmitted to variousmobile terminals 16 to the base station 14. The base station 14 may usethe channel quality indicators (CQIs) associated with the mobileterminals to schedule the data for transmission as well as selectappropriate coding and modulation for transmitting the scheduled data.The CQIs may be directly from the mobile terminals 16 or determined atthe base station 14 based on information provided by the mobileterminals 16. In either case, the CQI for each mobile terminal 16 is afunction of the degree to which the channel amplitude (or response)varies across the OFDM frequency band.

The scheduled data 44, which is a stream of bits, is scrambled in amanner reducing the peak-to-average power ratio associated with the datausing data scrambling logic 46. A cyclic redundancy check (CRC) for thescrambled data is determined and appended to the scrambled data usingCRC adding logic 48. Next, channel coding is performed using channelencoder logic 50 to effectively add redundancy to the data to facilitaterecovery and error correction at the mobile terminal 16. Again, thechannel coding for a particular mobile terminal 16 is based on the CQI.The channel encoder logic 50 uses known Turbo encoding techniques in oneembodiment. The encoded data is then processed by rate matching logic 52to compensate for the data expansion associated with encoding.

Bit interleaver logic 54 systematically recorders the bits in theencoded data to minimize the loss of consecutive data bits. Theresultant data bits are systematically napped into corresponding symbolsdepending on the chosen baseband modulation by mapping logic 56.Preferably, Quadrature Amplitude Modulation (QAM) or Quadrature PhaseShift Key (QPSK) modulation is used. The degree of modulation ispreferably chosen based on the CQI for the particular mobile terminal.The symbols may be systematically reordered to further, bolster theimmunity of the transmitted signal to periodic data loss caused byfrequency selective fading using symbol interleaver logic 58.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation.

Now referring specifically to FIG. 10A, for the dedicated pilotembodiment, the symbols are processed by S/P, SM layer mapping function63 which performs serial to parallel (S/P) conversion, and spatialmultiplexing (SM) layer mapping. The output of this process ismultiplied by the pre-coding matrix multiplier 65. A pilot sequence 69is also multiplied by the pre-coding matrix multiplier 65 using the samepre-coding matrix. The output of the pre-coding matrix multiplier 65 isinput to pilot and beacon pilot vectors insertion function 67.Non-pre-coded beacon pilot vectors 71 are also input to the pilot andbeacon pilot vectors insertion function 67. The pilots and beacon pilotvectors and data area then organized into two output streams, one perantenna.

Now referring specifically to FIG. 10B, for the common pilot embodiment,the symbols are processed by S/P, SM layer mapping function 63 whichperforms serial to parallel conversion, and spatial multiplexing layermapping. The output of this process is multiplied by the pre-codingmatrix multiplier 73. Beacon pilot vectors 77 are also multiplied by thepre-coding matrix multiplier 73 using the same pre-coding matrix. Theoutput of the pre-coding matrix multiplier 73 is input to pilot andbeacon pilot vectors insertion function 75. A non-pre-coded pilotsequence 79 is also input to the pilot and beacon pilot vectorsinsertion function 75. The pilots and beacon pilot vectors and data arethen organized into two output streams, one per antenna.

Referring again to both FIGS. 10A and 10B, each output stream is sent toa corresponding IFFT processor 62, illustrated separately for ease ofunderstanding. Those skilled in the art will recognize that one or moreprocessors may be used to provide such digital signal processing, aloneor in combination with other processing described herein. The IFFTprocessors 62 will operate on the respective symbols to provide aninverse Fourier Transform. The output of the IFFT processors 62 providessymbols in the time domain. The time domain symbols are grouped intoframes, which are associated with a prefix by prefix insertion logic 64.Each of the resultant signals is up-converted in the digital domain toan intermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) and digital-to-analog (D/A)conversion circuitry 66. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 68 and antennas 28. Notably, pilotsignals known by the intended mobile terminal 16 are scattered among thesub-carriers. The mobile terminal 16, which is discussed in detailbelow, will use the pilot signals for channel estimation.

Reference is now made to FIG. 11 to illustrate reception of thetransmitted signals by a mobile terminal 16. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal16, the respective signals fire demodulated and amplified bycorresponding RF circuitry 70. For the sake of conciseness and clarity,only one of the two receive paths is described and illustrated indetail. Analog-to-digital (A/D) converter and down-conversion circuitry72 digitizes and downconverts the analog signal for digital processing.The resultant digitized signal may be used by automatic gain controlcircuitry (AGC) 74 to control the gain of the amplifiers in the RFcircuitry 70 based on the received signal level.

Initially, the digitized signal is provided to synchronization logic 76,which includes coarse synchronization logic 78, which buffers severalOFDM symbols and calculates an auto-correlation between the twosuccessive OFDM symbols. A resultant time index corresponding to themaximum of the correlation result determines a fine synchronizationsearch window, which is used by fine synchronization logic 80 todetermine a precise framing starting position based on the headers. Theoutput of the fine synchronization logic 80 facilitates frameacquisition by frame alignment logic 84. Proper framing alignment isimportant so that subsequent FFT processing provides an accurateconversion from the time to the frequency domain. The finesynchronization algorithm is based on the correlation between thereceived pilot signals carried by the headers and a local copy of theknown pilot data. Once frame alignment acquisition occurs, the prefix ofthe OFDM symbol is removed with prefix removal logic 86 and resultantsamples are sent to frequency offset correction logic 88, whichcompensates for the system frequency offset caused by the unmatchedlocal oscillators in the transmitter and the receiver. Preferably, thesynchronization logic 76 includes frequency offset and clock estimationlogic 82, which is based on the headers to help estimate such effects onthe transmitted signal and provide those estimations to the correctionslogic 88 to properly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using FFT processing logic 90. Theresults are frequency domain symbols, which are sent to processing logic92. The processing logic 92 extracts the scattered pilot signal usingscattered pilot extraction logic 94, determines a channel estimate basedon the extracted pilot signal using channel estimation logic 96, andprovides channel responses for all sub-carriers using channelreconstruction logic 98. In order to determine a channel response foreach of the sub-carriers, the pilot signal is essentially multiple pilotsymbols that are scattered among the data symbols throughout the OFDMsub-carriers in a known pattern in both time and frequency. FIG. 12illustrates an exemplary scattering of pilot symbols among availablesub-carriers over a given time and frequency plot in an OFDMenvironment. Continuing with FIG. 11, the processing logic compares thereceived pilot symbols with the pilot symbols that are expected incertain sub-carriers at certain times to determine a channel responsefor the sub-carriers in which pilot symbols were transmitted. Theresults are interpolated to estimate a channel response for most, if notall, of the remaining sub-carriers for which pilot symbols were notprovided. The actual and interpolated channel responses are used toestimate an overall channel response, which includes the channelresponses for most, if not all, of the sub-carriers in the OFDM channel.

The frequency domain symbols and channel reconstruction information,which are derived from the channel responses for each receive path areprovided to an STC decoder 100, which provides STC decoding on bothreceived paths to recover the transmitted symbols. The channelreconstruction information provides equalization information to the STCdecoder 100 sufficient to remove the effects of the transmission channelwhen processing the respective frequency domain symbols

The recovered symbols are placed back in order using symbolde-interleaver logic 102, which corresponds to the symbol interleaverlogic 58 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 104. The bits are then de-interleaved using bit de-interleaverlogic 106, which corresponds to the bit interleaver logic 54 of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 108 and presented to channel decoder logic 110 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 112 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 114 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data 116.

FIG. 12 shows a block diagram of receiver components for performingbeacon pilot extraction, and pre-coding matrix detection. Thesecomponents can be added to a receiver such as shown in FIG. 11, and infact some components are shown in FIG. 12 that are in common with thoseof FIG. 11. The output of FFT 90 is processed by scattered pilotextraction 94, channel estimation and channel reconstruction 98. Theoutput of FFT 90 is also processed by beacon pilot extraction 400. Basedon the extracted beacon pilots, the pre-coding codeword matrix detectionis performed at 402. For the common pilots embodiment, this is fed backto the channel reconstruction function 98. Outputs of the channelestimation 96 and the pre-coding matrix detection 402 are input to adifferential codebook index search 404 which generates feedback that issent back to the transmitter.

What has been described is merely illustrative of the application of theprinciples of the invention. Other arrangements and methods can beimplemented by those skilled in the art without departing from thespirit and scope of the present invention.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A method of transmitting comprising: pre-coding each of at least twodata symbols using a respective pre-coding codeword to produce acorresponding plurality of pre-coded data symbols; transmitting arespective signal from each of a plurality of antennas, the respectivesignal comprising one of the pre-coded signals and at least one pilotfor use in channel estimation; the signals collectively furthercomprising at least one beacon pilot vector consisting of a respectivebeacon pilot per antenna, the beacon pilot vector containing contentsknown to a receiver for use by the receiver in determining the codewordused to pre-code the at least one data signal.
 2. The method of claim 1wherein each transmitted signal is an OFDM signal, and wherein for eachantenna the respective signal comprises a null in each sub-carrier andtime location used to transmit the at least one pilot in the respectivesignal of each other antenna.
 3. The method of claim 1 furthercomprising: pre-coding the pilots for use in channel estimation fortransmission; wherein the at least one beacon pilot vector istransmitted without pre-coding.
 4. The method of claim 1 wherein: thepilots for use in channel estimation are transmitted without pre-coding;the at least one beacon pilot vector is transmitted with pre-coding. 5.The method of claim 1 further comprising: receiving feedback indicatingwhich pre-coding codeword to use.
 6. The method of claim 1 wherein thefeedback comprises a differential codeword index.
 7. The method of claim3 comprising transmitting to a plurality of receivers with frequencydivision duplex (FDD) or time division duplex (TDD) separation betweencontent of different receivers; wherein pre-coding comprises using arespective pre-coding codeword for each receiver; the at least one pilotcomprises a respective at least one pilot dedicated to each receiverthat is pre-coded using the same codeword as the data for that receiver.8. The method of claim 4 comprising transmitting to a plurality ofreceivers with FDD or TDD separation between content of differentreceivers; wherein pre-coding comprises using a respective pre-codingcodeword for each receiver; the at least one pilot comprise pilots thatare for use by all receivers.
 9. The method of claim 1 performed foreach of a plurality of FDD or TDD MIMO radio resources.
 10. The methodof claim 9 wherein the FDD or TDD MIMO radio resources are OFDMresources.
 11. The method of claim 9 wherein said FDD or TDD MIMO radioresource is single carrier based.
 12. A method of receiving comprising:receiving a MIMO signal containing data symbols pre-coded with acodeword, the MIMO signal including pilots, and including at least onebeacon pilot vector containing contents known to a receiver, each beaconpilot vector containing one symbol from each transmit antenna;processing the at least one beacon pilot vector to determine whichcodeword was used to pre-code the data symbols.
 13. The method of claim12 further comprising: determining if the determined codeword was acodeword expected to be used.
 14. The method of claim 12 furthercomprising: comparing the determined codeword with an expected codeword;if there is a match between the determined codeword and the expectedcodeword, determining there is no error in codeword feedback, andperforming decoding.
 15. The method of claim 12 wherein the pilots arenot pre-coded and the at least one beacon pilot vector is pre-coded, andwherein processing the at least one beacon pilot vector to determinewhich codeword was used to pre-code the data comprises: performingchannel estimation using the pilots to produce channel estimates; usingthe known contents of the at least one beacon pilot vector, and thechannel estimates to determine which codeword was used.
 16. The methodof claim 12 wherein the pilots are also pre-coded and the at least onebeacon pilot vector is not pre-coded, and wherein processing at leastone beacon pilot vector to determine which codeword was used to pre-codethe data comprises: performing channel estimation using the pre-codedpilots to produce channel estimates; using the known contents of the atleast one beacon pilot vector, and the channel estimates to determinewhich codeword was used.
 17. The method of claim 12 further comprising:transmitting feedback indicating which codeword to use.
 18. The methodof claim 17 further comprising: comparing the determined codeword withthe codeword indicated by the feedback to determine if there has been apre-coding codeword feedback error.
 19. The method of claim 17 whereinthe feedback comprises a differential codeword index.
 20. The method ofclaim 16 further comprising tracking a channel of other receivers byprocessing the at least one un-precoded beacon pilot vector andpre-coded pilots of other receivers.
 21. The method of claim 18 furthercomprising: upon detecting that there is no pre-coding codeword feedbackerror, using received data for H-ARQ purposes; upon detecting that thereis a pre-coding codeword feedback error, using the determined codewordin place of the codeword indicated by the feedback.
 22. The method ofclaim 21 further comprising: transmitting the differential codewordfeedback based on the codeword determined using the at least one beaconpilot vector.
 23. A method comprising: in a transmitter, executing themethod of transmitting of claim 1; in a receiver: receiving a MIMOsignal containing data symbols pre-coded with a codeword, the MIMOsignal including pilots, and including at least one beacon pilot vectorcontaining contents known to a receiver, each beacon pilot vectorcontaining one symbol from each transmit antenna; processing the atleast one beacon pilot vector to determine which codeword was used topre-code the data symbols.
 24. A transmitter that executes the method ofclaim
 1. 25. A receiver that executes the method of claim
 12. 26. Amethod comprising: provisioning a frequency division duplex MIMO radioresource for facilitating detection of feedback errors; where saidresource includes at least one pilot for each transmit antenna; andwhere said provisioning includes pre-coding a known signal vector whensaid pilots are not pre-coded.
 27. A system comprising: a controlleroperable to: provision a frequency division duplex MIMO radio resourcefor facilitating detection of feedback errors; where said resourceincludes at least one pilot for each transmit antenna; and where saidcontroller is further operable to pre-code a known signal vector whensaid pilots are not pre-coded.
 28. A system comprising: a controlleroperable to: provision a frequency division duplex MIMO radio resourcefor facilitating detection of feedback errors; where said resourceincludes at least one pilot for each transmit antenna; and where saidcontroller is further operable to provision a non-pre-coded known signalvectors when said pilots are pre-coded.
 29. A system according to claim28 wherein said FDD MIMO radio resource is OFDM based.